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Inside your rifle

Discussion in 'Anything Airgun Related' started by Ballisticboy, Feb 7, 2014.

  1. Ballisticboy

    Ballisticboy Posting Addict

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    As some of you may remember I recently reported on using a model of a spring air rifle to look at the effects of reducing stroke length. I emphasised the limits to computer modelling and what I thought the model could be used for. For example, it cannot predict the effect of polishing and lubricating your rifle, but once done it could be used to predict how much preload needs to be reduced or what spring rate would work to bring the power to where it should be.
    One of the advantages of a computer model is that you get a lot of detail about what is happening inside the gun. When you pull the trigger the pellet leaves the gun about 10 milliseconds afterwards. The internals of the gun continue to move for about another 10 milliseconds afterwards. In this report I thought I would show some of the figures for a typical rifle. This will probably be of most interest to techies but hopefully to anyone interested in tuning their rifle or anyone who wonders what goes on inside their rifle.
    I have tried to use a fairly standard set up. The spring rate was 6000newtons/metre with a preload of 220 newtons. The piston mass was taken as 230grms with a 25mm diameter and an 81mm stroke. The transfer port was 3mm in diameter and 6mm long. The rifle was a .177 firing an 8.4grn pellet. All pretty normal stuff. What is not normal is that the model assumes there are absolutely no air leaks. I can put a leak into the system but for now I have assumed there is no loss of air.
    First a couple of the usual diagrams which I am sure many have seen before. The first diagram shows how the pressure inside the cylinder varies with time from when the piston starts to move to when the pellet leaves the barrel.

    The equivalent diagram for the piston movement is shown below.

    Both are normal though the size of the maximum pressure may surprise some at over 13.5 megapascals, around 2000psi in christian units. This means that the maximum force on your piston is around 1500lb. The piston is moving at over 30mph at its fastest. It does 0-30mph in 7.4 milliseconds and then 30-0mph in just over 1.5 milliseconds, both figures somewhat better than your car can do. When the piston stops it is just over 1mm away from the end of the cylinder. What is surprising is that the maximum pressure does not occur at the same time as the maximum piston travel, it occurs just before. This is because the pellet has already started to travel down the barrel giving the air more volume and hence eventually reducing the pressure.
    The air can be in the cylinder, the transfer port and the barrel. The variation in the percentage volume available in each is shown below.


    The transfer port never accounts for more than a small percentage of the volume available, just over 3.5% maximum. Contrary to many reports the transfer port does not appear to choke either. Choking occurs when the air velocity in the transfer port reaches the speed of sound. The maximum predicted velocity is just below 1050ft/sec but the speed of sound at this time is much higher due to the high gas temperature. The maximum port velocity occurs just before the maximum cylinder pressure.
    The barrel in this model was taken as 14.5 inches which is a bit short for maximum pellet speed. Below you can see how pellet speed varies with time.


    If you look carefully you can see that the pellet speed is still increasing at the time it gets to the end of the barrel. It needs about 16 inches to get to full speed in this case. The maximum pellet acceleration is nearly 29000g.
    So there it is, what happens inside the typical springer. Hope it is of interest to some people.
  2. JD

    JD Donator

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    To understand this better, We would need to know what spring would give 6000 newtons/metres,

    A standard Weihrauch or Titan spring perhaps and the number of coils it has.

    The maximum pellet acceleration is nearly 29000g, please can you explain what this figure means ?

    I take it, you've used a Mk1 .177 Hw77k as your model for this test ?
  3. Ballisticboy

    Ballisticboy Posting Addict

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    The spring is not meant to be any make in particular, it is just representative of many springs. A wire thickness of 3mm with an outside diameter of 20.5mm will give the correct spring rate with the 25 active coils used. The preload length is 37mm. There are a number of springs around close to this spec.
    29000g means it accelerates at 29000 times the acceleration due to gravity. If you were accelerated at the same rate you would weigh 29000 times what you weigh now. The HW77 is the basis but not necessarily the exact model. It is intended more as a general exercise to illustrate typical values within a 12fpe rifle. As was shown before, if the stroke is shortened the peak pressure and the efficiency will both increase.

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